Add support for WS-Discovery protocol about systemd

[WolfWings]

I'm happy to help work on this if it would be accepted, but as it's adding a new and somewhat complex protocol (XML, SOAP, another multicast port for resolved to have open, etc) I felt it best to ask if it would even be accepted first.

[Nardol]

As an alternative, there is WSDD-Native which also implements WSDD.
WSDD requires both LLMNR and HTTP as you stated just before, which maybe leave a resolver and responder scope?

[WolfWings]

WSDD doesn't actually use LLMNR at all, entirely unrelated ports and protocols. They overlap in purpose and both are checked (currently) on the Windows side, but yes it does use the HTTP wire format for queries/responses which was one reason I wasn't sure it was appropriate to include or not.

This third option for an existing natively packaged solution fits quite well though and yes integrates quite well with SystemD, I hadn't found it mentioned in previous web searches, closing RFE as resolved, thank you!